AFLOAT ON JAPAN'S INLAND SEA

By DONALD KEENE; Donald Keene is professor of Japanese at Columbia University and is the author of ''Dawn to the West: Japanese Literature in the Modern Era'' (Holt, Rinehart).

Published: October 6, 1985

No region of Japan gives more immediate pleasure than the Inland Sea. From the deck of the ferry or speedboat, patterns of islands change kaleidoscopically, now linking together to form a single serrated island, now standing apart as if to allow fishing boats and tankers to pass. If you are lucky with the weather, the brilliant green of the islands is set off by the sparkling blue of the sea and the softer blue of the sky, but even if it is raining, the varied shapes of the islands, dimly visible through the mist, claim your entire attention.

The Inland Sea is usually calm, more like a lake than a sea, because it is all but encircled by the main islands - Honshu to the north, and Shikoku and Kyushu to the south and west. The view is so entrancing that it's easy to forget that this is not simply a magnificent tourist attraction, but a much-used waterway dotted with entirely businesslike ships of many sizes and nationalities. The scene changes constantly, each turn on the way revealing another surprising cluster of islands and ships; but when I recently crossed on a large, luxurious motorboat, whose wall-to-wall carpeting extended over the ceiling and even around the television set, most of the other passengers were busily reading the sports pages. One can get accustomed to anything, even to the Inland Sea.

According to Japanese mythology, the islands of the Inland Sea are the oldest part of the country. In historical times the area around the Inland Sea was the scene of all three great battles of the civil war of the 12th century between the Heike and Genji clans, a bitter struggle that involved the whole country and ended with the total destruction of the once-omnipotent Heike. More recently, it was on an Inland Sea city, Hiroshima, that the first atomic bomb was dropped. The extraordinary postwar growth of the Japanese economy can be surmised from the huge industrial complexes along the coast, notably the immense automobile works outside Hiroshima where Mazda cars are manufactured.

In the past, travelers from China or elsewhere in Asia usually reached Japan by way of the Inland Sea. Their ships would pass through the Strait of Shimonoseki, where Honshu and Kyushu come close together, then make their way across the sea to Kobe or some other port. Nowadays travelers are much more likely to arrive by air in Tokyo or Osaka, and if they decide to visit the Inland Sea, they will start at the eastern, or Awaji, end rather than the Shimonoseki end.

Awaji, the largest of the more than 3,000 islands in this sea, is an appropriate place to begin. According to the ''Kojiki'' (Record of Ancient Matters) - the most sacred book of the Shinto religion, compiled in A.D. 712 - Japan was created by the copulation of the first two deities, Izanagi (the male) and Izanami (the female). Because they needed some firm ground for this process, they stood on the bridge of heaven and stirred the seas below with a spear until the island of Onogoro coagulated. They then descended to the island and set about their task, but were not satisfied with their progeny until the island of Awaji was born. Various small islands near Awaji have been identified as Onogoro, and in recent years an artifical island of that name has been constructed (by other means) near the fishing port of Tsuna, complete with a fun house, scary rides and the other amenities of an amusement park. Izanagi, who is not offered much reverence elsewhere in Japan, is wor-shiped at a Shinto shrine on Awaji because of his special relationship to the island.

The best way to reach Awaji is by a ferry from the city of Akashi, just west of Kobe. (The boat I rode on recently was called the Izanami; no doubt the company has another called Izanagi.) The bus ride along Awaji's east coast is hardly spectacular, but there are some agreeable fishing villages along the way. The most beautiful scenery is around Fukura, a town and bay at the island's southern tip, from which the Great Naruto Bridge now goes to the island of Shikoku.

Not far from the terminus of the bridge in Fukura is the Memorial Hall of the Great Naruto Bridge, which has exhibitions on different aspects of the Awaji culture. There is also a small theater where half-hour performances of puppet plays are staged for the benefit of busy tourists. The puppet theater of Awaji has somewhat different traditions from those of the Bunraku theater in Osaka: the plays are performed not by professional artists but by farmers and other amateurs.

The Great Naruto Bridge (completed last June) is probably a marvel of modern engineering, but to the uninformed eye it looks much like any other modern bridge that happens to be nearly 2,000 yards long. The most memorable sight is the uzu, or whirlpool, visible to the left about halfway across the bridge as you head toward Shikoku. It does not look very frightening, but its turbulence is often alluded to in Japanese literature. The view to the right of the bridge is also impressive: a first glimpse of the sweep of the Inland Sea with its islands rising like green mountain peaks from the water.